Numerous advances have been made in the efficiency with which still or moving picture information may be encoded, based upon the recognition that much information represented in a scene, in graphics or in text, is highly correlated. For example, it is possible to very accurately predict the intensity value (or other characteristics) of a particular portion of a picture, based perhaps upon spatial or temporal relationships, or other known properties of pictures, such as luminance/chrominance relations or by using velocity/displacement compensation. These predictions, in turn, are usually compared with the actual picture signal, so that only the error values need be encoded. With more sophisticated predictions, the error values statistically tend to be smaller, allowing further processing which takes advantage of the increased entropy; numerous techniques have been developed for this purpose. For example, the error signal can be non-uniformly quantized, so that inconsequential values are discarded or de-emphasized and important values emphasized. Since errors usually occur in spurts (near "non-predictable" edges, for example) various adaptive techniques can also be used, depending upon which area of the picture is being processed. Alternatively, the error signal can be processed to take advantage of spatial redundancy by a technique known as bit-plane encoding, wherein one or more of the most significant bits of each error sample, representing a group of contiguous picture elements, are coded as a series of run lengths, rather than individually. However, it is noted that the lower order bits of each error value are not usually highly correlated, reducing the effectiveness of the bit-plane approach.
Despite the improvements which the foregoing techniques provide, it nevertheless remains that the efficiency with which multi-level signals having a statistically "preferred" value are encoded may be increased, by reducing redundancy still further and thus reducing the amount of data needed to represent the signal. Accordingly, the broad object of the present invention is to enable more efficient encoding of a multi-level signal wherein one particular level or value occurs much more frequently than any other value. One such multi-level signal very well suited for encoding in accordance with the present invention is an error signal derived from a predictive picture encoder.